PhD student in spider behavioural ecology and evolution

It’s about sexual conflict and it’s about time

I had wanted to talk about something else for my first blog post because talking about the background to my own work seems a bit indulgent. But first semester has already rolled around and I still haven’t posted anything. More importantly, the Entomological Society of New Zealand conference is coming up after Easter and I’ve told enough people that I’m going to present that I’m now obliged to submit an abstract. The last time I looked at my dissertation I was swaddled in a giraffe suit and hooked up to a tea IV drip.

So, if nothing else, researching for this post is a good way for me to remind myself about what the hell my dissertation was all about and shoe horn it back into my brain in time for the abstract submission on the 16th (been and past. Status: Accepted. Holla.)

By the way, my dissertation was about sexual cannibalism in praying mantises. But as exciting and bizarre as sexual cannibalism sounds, it’s only one of a spectrum of…unpleasantness between the sexes.

To take humans as a familiar example. It hardly needs saying that interactions between men and women throughout the world are fraught with direct conflict, misunderstandings and incompatibility of expectations. Women want ‘the one’ while men supposedly want ‘one right now’. It’s cliché to state it plainly but these tropes do have a basis in biological fact (whether they should be enforced in media is another kettle of fish.) Anisogamy is the phenomenon in which the gametes, the bodies that carry a parent’s genetic contribution to a child (i.e. female eggs and male sperm), differ in size between the sexes. Females provide a much larger, well provisioned gamete but producing only a small total in their life-times. Eventually, women reach menopause which heralds the end of any reproductive behaviour.

Meanwhile sperm, relative to eggs, are tiny. But while they may be tiny, they

So meta

are also produced at a rate of 1,500 spermatozoa per second and men will continue to produce sperm well into their twilight years, long after their female counterparts have lost any reproductive capacity. So on an individual, national, global scale, the number of women’s eggs is minute relative to the number of available sperm. In short, ova are a limited resource.

This pattern of ova paucity and sperm surplus holds in the majority of animals, not just humans, and the consequence of this is that it is in a female’s interest to be choosy about who she mates with in order to ‘make the most’ of her limited number of offspring. Males, meanwhile, have no such limit and therefore tend to improve their ‘reproductive fitness’ by fertilising as many eggs as possible with his veritable bounty of sperm. This disjunction of ‘optimal strategies’ for males and females generates ‘sexual conflict’(1). This could be that males are more inclined to copulate with as many females as possible whether those female’s want to or not.

And you can see how such conflict arises. A surplus of sperm relative to the number of eggs means that it is therefore advantageous for males, overtime, to develop behaviours or structures to force copulation on females and, in response should these behaviours become prevalent, becomes advantageous for females to develop behaviours and structures which help her to evade male ‘affections’.

The humble Gerris incognitus

Possibly one of the simplest and best understood examples of this is the that of the humble water strider (2). To state it simply, male water striders cling to a female’s back and develop genital clasping structures to hold on to the struggling female beneath him. Females are not only forced to copulate but they’re also forced to carry the male around on her back. If this male behaviour becomes common, it becomes advantageous for females to kick males off their backs and to develop uncomfortable looking spines on their abdomens to discourage their suitors from holding on too long.

Water striders are just one example of sexual conflict and manifestations of this conflict are extremely common throughout the animal kingdom. What differs is the extent to which this conflict has become exaggerated. The water striders example with its repeated to-ing and fro-ing of behaviours and structural changes has granted that particular system the descriptor of ‘sexually antagonistic coevolution’. Coevolution refers to when groups evolve with respect to each other rather than due to selective pressures enforced by their environment. The development of close knit relationships between certain flower and bee species is a good example of a mutually beneficial form of coevolution. SAC is obviously a little less congenial.

From ‘Franken franby Katsuhisa Kigitsu. Censored by moi.

So next time: Mantises eating mantises to make more mantises.

References:

1) The concept of sexual conflict was first formally described and related to anisogamy by Parker in 1979 in his seminal work, ‘Sexual selection and sexual conflict’, a chapter within Sexual selection and reproductive competition in insects, which was edited by Murray and Nancy Blum.

2) The understanding of the sexually antagonistic coevolution within the water strider species Gerris incognitus has been developed over the last two decades with some of the most important papers being published by Arnqvist and Rowe in 1995 and in 2002.